Industrial Field of the Invention
The present invention relates to an optical material; more specifically it relates to an optical material having excellent properties of optical transmissivity, heat resistance and humidity resistance.
Related Art
Glass or high-molecular-weight substances are generally used as a material for optical recording media in a compact disk, a video disk, a computer disk, etc.; and plastic materials are desirable for a mass-production in view of easiness in a mold processing. In this case, the plastic materials are required to have those properties: (1) optical properties of a good transparency and a high light transmissivity and also of a stable refractive index and a small birefringent index; (2) chemical properties of a good adherence to an optical reflection layer of aluminum, silver or the like, and further of a large protective strength against corrosion; (3) physical properties of strength necessary as a substrate and of resistance enough to a thermal deformation; (4) a good mold-processability, etc.
At present, it is polycarbonate, polymethyl methacrylate, etc., that are used as a plastic material which can comparatively satisfy the above requirements. However, polycarbonate has somewhat high hygroscopicity, is likely to cause birefringence and undergoes hydrolysis due to its molecular structure, although it has good heat resistance since its glass transition point (T.sub.g) is high. On the other hand, polymethyl methacrylate causes a corrosion on an optical reflection layer due to its high hygroscopicity and a warpage on a disk surface with a change in dimension and form, in spite of its optical properties of a good transparency and a small refringent index.
The present inventors have made a diligent study to overcome the above defects, and have consequently completed the present invention by finding that a conjugated diene polymer cyclization product having a cyclization ratio of not less than 80% or a hydrogenated product thereof has such properties suitable for an optical material as optical transmissivity, mechanical strength, heat resistance, humidity resistance, etc.